1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus using an optical recording medium having a plurality of tracks arranged at a predetermined pitch, such as an optical disk or an optical card, and to detection of digression of a light beam from a target track during tracking servo control, i.e., off-track.
2. Description of the Related Art
Generally, in such a recording/reproducing apparatus, if an off-track state occurs during reproduction, a light beam scans a track which is different from a target track from which data should be reproduced. As a result, different data is reproduced in the course of the reproduction. If an off-track state occurs during a recording operation, data is recorded on a track which is different from a target track on which the data should be recorded. In this case, if the light beam digresses to a track on which data is already recorded, the recorded data is destroyed. During a recording/reproducing operation, therefore, an off-track or on-track state must be checked while tracking servo control is performed so as to scan a laser beam along the center line of the target track. When an off-track state is detected, the recording/reproducing operation is stopped and the laser beam is returned to the center line of the target track, thus preventing erroneous recording on another track during recording.
A digression detecting circuit for detecting an off-track state is disclosed in U.S. Pat. No. 4,554,652. This conventional digression detecting circuit determines off-track by using a track-error signal which is used for tracking servo control for moving an objective lens in a direction perpendicular to a track during scanning of the track by a laser beam and is increased or decreased when the difference between the center of the track and that of a laser beam spot is increased. The track-error signal is supplied first to a low-pass filter so as to cut a high-frequency component. A DC component is then removed from the signal by a DC-cut circuit. The signal upon removal of these components is compared with the upper and lower threshold values of a window comparator. If this signal is larger/smaller than the upper/lower threshold value, an off-track signal is output.
In this case, if dust or damage, such as a scratch, is present on a recording medium, such as an optical disk or an optical card, and a beam spot scans it, a track-error signal is disturbed in spite of an on-track state, and its level may become higher or lower than the upper or lower threshold value. As a result, this state may not be discriminated from a true off-track state, and hence an off-track state may be erroneously detected.
Note that if a rotary recording medium, such as an optical disk, is used, the jitter frequency component of a track-error signal due to dust or damage, such as a scratch, becomes high because the scanning speed of a light beam during a recording/reproducing operation is high, and hence the jitter component can be removed to a certain degree by a low-pass filter. Therefore, serious problem is not posed even in the above-described conventional apparatus. Furthermore, in an optical disk and the like, a protective layer on the surface of a recording medium is relatively thick, so that the size of a beam spot on the protective layer is larger than that of a beam spot on the recording medium layer due to refraction. Therefore, the dust or damage is very small with reference to a beam spot in an optical disk. For this reason, the influence of dust is reduced.
In an optical card, however, erroneous detection due to dust or damage, such as a scratch, poses a problem for the following reasons. In the optical card, the scanning speed of a light beam cannot be excessively increased because tracks are linearly formed. Therefore, the jitter frequency component of a track-error signal due to dust, a scratch, or the like, becomes low and differs less from the jitter frequency component of a track-error signal during an off-track period. For this reason, it is difficult to prevent erroneous detection due to the influence of dust, a scratch, or the like by simply removing a high-frequency component from a track-error signal using a low-pass filter. In addition, since the optical card is often carried by a user, it is more susceptible to damage by dust, a scratch, or the like than the optical disk. Moreover, the thickness of the protective layer of the optical card is decreased in order to increase portability, so that the size of a beam spot on the protective layer does not significantly differ from that of a beam spot on the recording medium layer. Therefore, the optical card is susceptible to the influence of dust, a scratch, or the like.